The present invention relates to systems and methods for aligning a portable device with an inductive wireless power supply, and more particularly to such systems that use magnetic attraction to align a portable device on an induction charging surface.
Inductive wireless power supply systems include an inductive power supply with a primary coil and a portable device with a secondary coil. The inductive power supply may also include an inductive charging surface for placement of the portable device. In a typical situation involving this configuration, the portable device is placed on the inductive charging surface so that the primary coil and secondary coil are aligned and may inductively couple for wireless power transfer.
In some applications, attempts have been made to place the secondary coil in close alignment and proximity to the primary coil located adjacent to the inductive charging surface. Alignment and proximity may affect the mutual inductance between the primary coil and the secondary coil, which influences the efficiency of the power transfer. As used in the description, the term alignment pertains to the concentricity of the primary coil and secondary coil, and the term proximity relates to the planar spacing between the primary coil and secondary coil.
A user is often unaware of the exact location of the primary or secondary coil within the inductive power supply or portable electronic device. The secondary coil and primary coil may not be exposed so that the user knows their position within the portable electronic device or the inductive wireless power supply. Absent some additional information, the user may find it difficult to achieve consistent alignment between the primary and secondary coils and therefore efficient power transfer. As a result, many conventional systems and methods have attempted to improve the user's ability to provide close alignment of the secondary coil relative to the primary coil.
Some examples of conventional systems and methods for facilitating proper alignment of a primary coil with a secondary coil include geometrically matched surfaces, permanent magnets, magnetic attractors, multiple coil arrays, nested coils, and movable coils. These systems and methods may be designed to create improved mutual inductance between primary coils and secondary coils through close alignment.
An inductive wireless power supply that uses multiple coil arrays may allow the user to place the portable electronic device near the inductive power supply without concern for a specific location or close alignment. The multiple coil arrays may include more than one primary coil located in different areas of the inductive charging surface so that any of the primary coils may be selected to wirelessly power a portable electronic device. Accordingly, the surface area over which a secondary coil may be placed in close alignment with at least one primary coil may be increased, which may free the user from having to know the position of the secondary coil relative to a primary coil. However, multiple coils arrays tend to be expensive to implement, which in many cases makes them an inappropriate solution for achieving close alignment between a primary coil and a secondary coil.
In inductive wireless power supplies that use movable coils, the user may also place the portable electronic device near the inductive wireless power supply without concern for a specific location or close alignment. Accordingly, the inductive wireless power supply of this example also allows for spatial freedom. Movable coil systems in general include a primary coil that may change positions within the inductive power supply to facilitate alignment with a secondary coil. In many applications, actuators or motors may be utilized to move the primary coil based on a sensed location of the secondary coil. As a result of this physical movement used in the movable coil system, components may be prone to mechanical failure.
Nested coils, such as one or more coils nested within another coil, may limit the interoperability of the inductive charger. Specifically, although the nested coil solution may provide some spatial freedom, the inductive charger and portable device may use a specific nest geometry for operating with each other.
In another example, the inductive power supply includes permanent magnets or magnetic attractors to improve alignment between the primary coil and secondary coil. The permanent magnet or magnetic attractor may be associated with the primary coil and secondary coil to produce magnetic force. For example, the primary coil and secondary coil may each have permanent magnets that attract to each other. In another example, either the primary coil or secondary coil may have a permanent magnet used to attract the other coil having a magnetic attractor, which is a slug of ferromagnetic material in the other coil. Accordingly, permanent magnets can be utilized to attract the portable electronic device to the inductive power supply through magnetic force. Using this force, the system may aid the user to align the secondary coil relative to the primary coil.
The use of permanent magnets to achieve alignment and proximity in this example system may not exist without certain limitations. First, permanent magnets may heat up in the presence of an AC magnetic field, causing undesired heat transfer to nearby components. Second, the amount of force used to align the portable electronic device with the inductive power supply may be large. Larger magnetic forces can correlate to larger permanent magnets, and space within the portable electronic device or the inductive power supply may be limited. Permanent magnets also can be expensive. Third, permanent magnets may attract surrounding objects, such as paper clips, which can cause a poor user experience with the inductive wireless power system. A fourth limitation may be the DC magnetic flux produced from the permanent magnets. The inductive performance of a system may be degraded by the presence of permanent magnets in close proximity to the inductive coils and flux guides. Further, the presence of DC magnetic flux can lower the saturation point of magnetic shielding materials. These magnetic shielding materials may be used to guide the AC flux of the inductive charging system, and a lower saturation point means that more AC flux may be lost.
Lastly, balancing characteristics of permanent magnets may include performance trade-offs. For example, magnets that are too weak might not affect alignment or be perceptible to a user. On the other hand, magnets that are too strong may result in the inductive charger being lifted off the table when the portable device is picked up.